1. Field of the Invention
The present invention relates to a light guide plate used in a lighting apparatus such as a backlight of a liquid crystal display, a lighting apparatus and a liquid crystal display provided with the light guide plate, and a light guide plate molding die.
2. Description of the Prior Art
According to a known backlight of a liquid crystal display, a reflection sheet is provided on a reflection side of a light guide plate, and a prism sheet and a diffusion sheet or a polarized light reflection sheet are laminated on a light emission side of the light guide plate. Light input from a side of the light guide plate is uniformly emitted from light emission surface of the light guide plate to uniformly irradiate a liquid crystal display panel provided in front of the diffusion sheet or the polarized light reflection sheet. According to this light guide plate, protrusions are formed on the reflection surface (on the side of the reflection sheet) and the light emission surface with predetermined surface roughness to adjust direction and spread, or luminance and uniformity, of the light emitted from the light emission surface of the light guide plate.
Japanese Unexamined Patent Publication No. 10-188642 discloses a light guide plate, for use in a lighting apparatus for performing surface illumination such as a backlight, having a reflection pattern which reflects light diffusely, in which a first reflection pattern is provided on one surface of the light guide plate and a second reflection pattern is provided so as not to overlap with the first reflection pattern on the other surface of the light guide plate. Japanese Unexamined Patent Publication No. 10-188642 further discloses that, because the reflection patterns are provided so as not to overlap with each other, light diffused and reflected by each reflection pattern is emitted from slits in the reflection pattern on the opposite side surface, so that illumination can be performed from both sides of the one light guide plate.
Japanese Unexamined Patent Publication No. 2002-169033 discloses a directional light guide plate in which light from an end face is emitted from each of the front and the back surfaces of the light guide plate, in which many fine spherical protrusions are provided on a side of a light emission surface, in which a prism is continuously provided on the opposed surface in the direction perpendicular to the light emission surface, and in which a diffusion protrusion pattern having small angular protrusions are superimposed in a region having relatively low luminance on the light emission surface. Japanese Unexamined Patent Publication No. 2002-169033 discloses that with this directional light guide plate, a uniform and high-luminance surface light source for a large screen is achieved.
The known light guide plate is molded by injection molding, and the protrusions are formed by transferring recesses formed on a molding surface of an insert die provided in the die. The recesses of the insert die are formed by a sand blasting method or other suitable method. More specifically, first, the entire surface of the insert die is lightly sandblasted to adjust the luminance and the uniformity characteristics of the light guide plate to be molded, and then, the portion of the insert die in which surface roughness is needed to be rougher is sandblasted again.
The recesses are formed in portions of the insert die in which the surface roughness of the insert die corresponds to the surface roughness of the light guide plate, which is a molded product. However, in the case where the recesses are formed by the method described above, when light is emitted from the molded light guide plate, luminance non-uniformity is created at a boundary portion in which the surface roughness changes, which appears on a display screen of the liquid crystal display.
Japanese Unexamined Patent Publication No. 2003-39487 discloses a method for forming a gradation pattern of protrusions on a molding surface of an insert die of a light guide plate molding die by projecting a shot blasting material into strips on the surface of the insert die of the light guide plate molding die at a controlled pressure in parallel. The difference in protrusion forming pressure variation is not to be more than 0.02 MPa in the portion between adjacent blast bands having protrusions with different diameters and depths. Japanese Unexamined Patent Publication No. 2003-39487 discloses that non-uniformity is not created at a boundary portion between the blast bands because of the gradation pattern by the protrusions.